To find the time required to screen-print a batch of 6 shirts, substitute \( s = 6 \) into the equation:
\[ t = s + 3 \]
Substituting 6 for \( s \):
\[ t = 6 + 3 \]
Now, calculate:
\[ t = 9 \]
Therefore, it takes 9 minutes to screen-print a batch of 6 shirts.